Substrate holders, such as electrostatic clamps, are widely deployed in apparatus that impart heat into substrates, which may require controlled heat transfer into or out of the substrate holder to maintain the proper substrate temperature. The heat may be imparted from a process itself or by deliberate heating of the substrate. In resistively heated electrostatic clamps, gas may be provided between a heating block and a cooled base in order to aid thermal transfer. Because the heating block and base may comprise dissimilar materials, such as a ceramic and a metal, respectively, it may be necessary to avoid bonding the two components together to avoid excessive thermal mismatch strains when the block is heated. The use of gas to transfer heat from the heating block may therefore be necessary since thermal transfer may be very low in a low pressure ambient if the base is not bonded to the heating block. The temperature mismatch between the ceramic and base may be reduced by using a high enough pressure of gas to rapidly transfer heat away from the ceramic. However, the gas supplied between ceramic and metal may leak along the interface between base and heater block and into a process chamber containing the electrostatic clamp. The unwanted gas leakage may lead to poor process control or substrate contamination in processes that depend on control of the gas ambient in the process chamber, including plasma or beamline implantation processes in ion implanters.
FIG. 1a depicts a prior art ESC configuration 10 in which a base 12 and heating block 14 are joined together. ESC 10 includes a heater (not shown), which may be used to resistively heat substrates that are supported by the heating block 104. ESC 10 may operate as a substrate holder in a process chamber, such as a low pressure chamber for performing one or more processes on the substrate. Examples of such low pressure chambers include plasma and ion beam tools, which may be evacuated to a pressure of 10−7 Torr or less before substrate processing and may operate in an ambient gas pressure in the range of 10−7-100 torr, for example.
During processing, substrates 16 may be heated to a fixed temperature using heating block 14. In order to maintain process control, base 12 may act as a heat sink to maintain proper heat flow out of heating block 14, and thereby more accurately control substrate temperature, as well as temperature in the heating block. In order to provide appropriate heat conduction between heating block 14 and base 12, a gas may provided through an inlet (not shown) into a narrow gap (chamber) 18 formed between heating block 14 and base 12. The gas may aide in thermal conduction to maintain a rapid heat flow into base 12. This configuration also helps avoid thermal mismatch problems between base 12 and heating block 14 that may occur between the base and heating block, as noted above.
However, the prior art ESC configuration of FIG. 1 may result in gas leaks into the process chamber 24 outside of ESC 10. For example, gas may leak along interface 20 located between heating block 14 and base 12 that is located towards the outside of gap 18. Because heating block 14 may be a ceramic and base 12 may be a metal, the interfaces may move with respect to one another during heating. In addition, the dissimilar materials may not form an intimate contact at their mutual interface, leading to appreciable leakage of gas in the direction 22. For example, the pressure in gap 18 may be several tens or of Torr or higher and the pressure outside ESC 10 may be in the mTorr range or less, which large pressure differential, combined with the imperfect seal at interface 22, may cause a large leak rate of gas into the substrate processing chamber 24.
Concomitant with gas leakage, the gas pressure may vary across the gap 18, as illustrated in FIG. 1b, leading to temperature non-uniformities across ESC 10. The pressure may be highest near the center of the ESC at point I, which may be located near an inlet of gas (not shown) provided to the gap. The pressure may steadily drop toward the outer portion of the gap 18 (RC) and then rapidly drop across the nominal sealing surface 22 to the outside edge RO of the ESC, as gas leaks out of gap 18. This varying pressure may result in a temperature gradient along the x-direction as the rate of heat conduction from heater block 14 to base 12 varies.
It will be apparent therefore that improvements are desirable over known ESC configurations used for heating substrates.